An alternator includes, in a manner known per se, a field winding, generally at the rotor, supplied with DC current either by rings and brushes or by an exciter, so as to generate an AC voltage in an armature winding, generally at the stator.
There are several known solutions for delivering a DC voltage from an alternator operating at variable speed.
A first solution, illustrated in FIG. 1, consists in rectifying the AC voltage delivered by the alternator 10 by means of a simple diode bridge 11. The excitation of the rotor of the alternator by means of an exciter 12 or by an assembly composed of rings and brushes is continuously adjusted by a regulator 13 in order to maintain a constant DC voltage regardless of the speed of the driving element and the power to be delivered.
This simple and robust solution is only able to be used when the range of variation of speed and of power is relatively low. For example, for a generator set in which the power of the combustion engine decreases in terms of N3, the range of variation of rotational speed is typically from 75% to 120% of the nominal rotational speed. Expanding this speed range downward involves significant oversizing of the alternator, thereby increasing the cost thereof.
A second solution, illustrated in FIG. 2, consists in rectifying the AC voltage delivered by the alternator 10 by means of an active rectifier 15 composed of an IGBT inverter, controlled by pulse width modulation. For certain operating points, in particular those for which the excitation current of the revolving that is required for obtaining the desired DC voltage does not lead to saturation of the magnetic circuit, the transistors 16 of the inverter are not controlled, and the rectification is carried out in a manner identical to that described for solution 1. For the other operating points, the excitation current of the revolving field is kept fixed and the transistors of the inverter are driven in order to step up the voltage to the desired level. The publication WO 2012/110979 A1 describes, in particular, a means for arbitrating between the two types of operation in order to minimize the level of overall losses from the system.
This solution enables operation over a wide speed range, for example from 40% to 120% of the nominal speed, but nevertheless has the following drawbacks: